专栏文章提交时出现 ErrorException

[storefee]

$data = array_merge([
            'topic_type' => $topic_type,
            'topic_link' => $topic->link(),
            'topic_title' => $topic->title,
            'topic_category_id' => $topic->category->id,
            'topic_category_name' => $topic->category->name,
        ], $extra_data);

报出这一行 'topic_category_id' => $topic->category->id,出问题，

ErrorException in BaseActivity.php line 42:
Trying to get property of non-object

应该是$topic->category 为空的问题。